First of all, Pashmee, there is no such thing as permanent writer's block. Writer's block can always be overcome. You just have to apply a few tricks and have the desire to do so.
As for lyrics or music first, I personally have worked with a lot of bands over the years and I got to see many different approaches to writing music. In one band the singer would write lyrics and a melody, bring it to the band and sing it, then the guitarist would flesh out the chords. Another band would all write together, just jamming until something came out, with the singer scribbling lyrics as they came to him. Another songwriter I know would just record himself jamming for a half hour on whatever intstrument he chose, then go back and listen to the tape, picking out parts he could use. Etc, etc, etc... The point is, I took note of all these different approaches and classified them in my mind. Now when I sit down to write a song, I'll just choose a method that I've classified. This gives each song I write a unique flavor.